The present invention relates to a luminaire (i.e., the complete lighting unit); and more particularly, it relates to a luminaire which may be used for indirect lighting particularly in large open areas such as are common-place in modern offices. As will be further explained below, one of the advantages of the invention is that its use is not limited only to open office areas, but rather, it can be carried through to the lighting of hallways, executive suites or conference rooms. However, the invention is particularly suited to provide illumination for typical office situations with efficient use of energy while having those characteristics which recent research has shown to be desirable from a psychological viewpoint.
Modern offices are characterized by large open spaces having a ceiling height normally about eight feet, but which may extend to nine feet. The space may be sectioned for privacy by movable wall partitions. These partitions normally have a height of about five to seven feet.
In considering the lighting requirements for an office, among the more important considerations are the need for individual task lighting, for example, for surfaces approximately two and half feet above the floor, and the need to provide some ambient lighting. The latter is desirable both from a safety standpoint and for psychological reasons, as will be discussed below.
Direct lighting as a source of lighting for offices provides high luminance for task areas, but in general, the quality of lighting is not good in terms of visibility and visual comfort of a person.
Lenses have been used in luminaires employing fluorescent lamps for direct lighting. One type of lens employs individual conical or spherical lenses in a repeating pattern. Another uses parabolic reflectors to direct light downwardly. In both of these systems, a light transmission pattern is created in which the light is directed downwardly and generally confined within an angle of about 45 degrees from the vertical. Further, neither of these systems provides ambient lighting for vertical surfaces which has been found to be a distinct preference and produces feelings of spaciousness and comfort.
Indirect lighting obviates the problem of glare perceived by an observer, but most indirect lighting systems have the disadvantage of collecting dust which gathers on the lamps and any reflectors beneath the lamps, and this is a major source of reduced luminance. Another disadvantage of most indirect lighting systems is a psychological one. Research has shown that human beings have a distinct preference for lighter rooms which are uniformly illuminated. In other words, a person may comfortably perform a task in an area at a relatively low luminance level provided there is a minimum of light available and the luminance pattern does not change substantially within his field of view. If the luminance level is substantially lower than that of the area from which a person enters, there will be a period of adaptation, but once the adjustment has been effected, there is little difficulty in performing tasks if the illumination meets the minimum requirements for the task. Further, people prefer to be able to see the source of light. It provides a sense of perspective and has been found to be a factor to be considered in providing office lighting. As indicated, most indirect lighting systems have as a principal object the generation of light while masking completely the origin of the light.
Lenses have in the past been placed above a source of light. Some of these lenses do not generate a pattern of illuminance above the luminaire which is uniform. Either a very bright spot is created above the luminaire (the more common characteristic), or a dark area is generated. In either case, the substantial variation in the illumination pattern on the ceiling is easily perceived and is undesirable. Further, in the case of the bright spot, inefficiency results because most of the light is reflected back to the luminaire rather than being used to light the room.
If a dark area is created on the ceiling, it is undesirable from an aesthetic as well as a psychological viewpoint because of the preferences mentioned above.
The luminaire of the present invention has as a principal object the provision of indirect lighting of large open office systems by achieving a uniform distribution of light (i.e. illumination pattern) on a ceiling over the luminaire as well as by fanning the light out laterally to extend the uniform luminance on the ceiling to adjacent areas not directly above the luminaire. Further, due to the reinforcement and bending of lateral rays, vertical surfaces are illuminated even at a substantial distance from the unit.
The luminaire includes an opaque housing beneath the source of light which preferably includes two or three fluorescent lamps placed side by side. The length of the lamps helps to distribute the light along the length of the housing.
A lens is mounted to the housing above the source, and in the illustrated embodiment, it includes a generally flat upper portion and lateral portions which curve downwardly and join to the housing to enclose the lamps. The upper portion of the lens includes a flat upper surface and a series of light-diverging prisms on the lower surface, facing the source of light. Adjacent prisms are spaced by a flat portion. The function of the prisms is to refract and diverge incident light, and the function of the flats is to permit incident light to pass between the prisms. The prisms and flats cooperate to provide a uniform pattern of luminance on a ceiling above the luminaire which normally has a high reflectance to provide good indirect lighting qualities. This uniformity is effective when the ceiling is placed about two to three feet above the upper surface of the lens and the effect persists even though the spacing is reduced to one foot.
A reflector is located beneath the light sources in the housing to direct light from the sources out to the lateral portions of the lens. Some of the reflected light is transmitted upwardly to the upper portion of the lens. In the illustrated embodiment there are two lateral lens portions, but only one is necessary, for example, if the luminaire is designed for placement directly on the side of a vertical wall. The lateral lens portion or portions are curved downwardly from the top lens portion to the upper edge of the housing, and they contain a plurality of external prisms extending the length of the lens.
The placement of the source of light in the housing is such that it cannot be seen directly by an observer having an eye level the same height as or slightly above the source. Further, because of the interposition of the lens, the light source cannot be seen directly from any elevation. The lateral lens portions bend the light from the source as well as the light from the reflectors toward the horizontal to "fan" the incident light and thereby extend the uniform pattern of luminance onto portions of the ceiling not directly above the luminaire. A pattern of luminance having substantial uniformity is thus achieved on the ceiling directly over-head and extending a few feet either side of the lens without creating bright spots which cause glare or dark areas on the ceiling, as characterized by some prior luminaires used for indirect lighting. Thus, luminaires of the present invention may be spaced at relatively wide distances if they are associated, for example, with movable wall panels since adjacent luminaires will both exhibit this fanning effect and achieve a luminance for indirect lighting which not only provides uniform ambient lighting but also more than adequate task lighting at the same time.
The fact that the light flux density diminishes as the distance between the luminaire and the point being illuminated increases as well as the fact that the flux density is reduced because the flux density in a plane perpendicular to the source is spread over an extended area for large angles of incidence (i.e. the angle relative to the normal), as is the case for areas not directly above the luminaire, are compensated by the effect of the reflector which routes most of the light from the bottom of the source toward the lateral lens portions, although some is transmitted to the top lens portion because of the size of the lamps. This, as well as the particular prism designs, disclosed below, cooperates to achieve a pattern of substantially uniform luminance which is highly desirable from a psychological standpoint and which achieves uniform task light as well as ambient lighting while reducing the amount of electricity to accomplish this in a large open office area.
The lateral lens portions of the luminaire transmit direct and reflected light from the source to illuminate vertical surfaces such as walls and provide a brighter ambient lighting.
Further, because the prisms on the curved lateral portion of the lens are on the exterior surface of the lens, they act to segment the incident light and provide a series of softly glowing lines of light which are perceptible to a person standing to the side of the luminaire, but without transmitting a direct image of the source. Thus, persons in a space illuminated by the present invention have the psychological benefit of knowing where the light is coming from, yet they do not experience the discomfort of glare which accompanies normal direct lighting. When the luminaire of the present invention is mounted to the top of a movable wall panel at a height of 72 to 84 inches, the light source is out of the direct line of sight of a person less than about seven feet tall. Nevertheless, he is capable of seeing light emanating from the exit surfaces of the prisms on the curved lateral portions of the lens. A small amount of light also emanates from the riser surfaces of the prisms through internal reflection, and this light is greatly diminished in luminance, but it extends downwardly from the luminaire, thereby providing a visual image of the source of light even to a person sitting at a work surface mounted to the same or adjacent wall panel to which the luminaire is mounted.
Another advantage to the present invention is that the lens provides a dust cover for the lamps, and the smooth exterior surface of the upper portion of the lens facilitates cleaning. That is, are no crevices in which dust can gather. Thus, a major source of reduction of luminance in indirect lighting is obviated, namely, the collection of dust on the lens or lamps.
Other features and advantages of the invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.